1. Field of the Invention
This application relates to the field of detection of chemical substances and, more particularly, to an ion source for providing ions to a chemical detector.
2. Description of Related Art
Chemical detection may be performed by a variety of detection instruments, such as a gas chromatograph, an ion mobility spectrometer, a mass spectrometer, and/or a differential mobility spectrometer. Many of these chemical detectors require that a chemical gas sample (the “sample gas”) be ionized in an ion source prior to reaching the detection component. In many embodiments the ion source to ionize the sample gas is operated at atmospheric pressure (the “atmospheric ion source”).
A variety of methods for ionizing the sample gas are known. A radioactive source, such as an alpha or beta source, may be employed. X-ray sources are known (see, for example, U.S. Pat. No. 6,429,426 entitled “Ionization chamber with electron source”, U.S. Pat. No. 6,740,874 entitled “Ion mobility spectrometer with mechanically stabilized vacuum-tight x-ray window”, and U.S. Pat. No. 6,586,729 entitled “Ion mobility spectrometer with non-radioactive ion source, all to Doring) in addition to photoelectric ion sources (see, for example, U.S. Pat. No. 7,304,298 to Swenson, et al., entitled “Photoemissive ion mobility spectrometry in ambient air” and U.S. Pat. No. 5,300,773 to Davies, entitled “Pulsed ionization ion mobility sensor”), ultraviolet lamp ion sources (see, for example, U.S. Pat. No. 7,002,146 to Fischer, et al., entitled “Ion sampling for APPI mass spectrometry”), and several kinds of corona sources. Corona sources are typically either continuous (DC) or radio frequency (AC) (see, for example, U.S. Pat. No. 7,057,130 to Gefter, et al., entitled “Ion generation method and apparatus”, U.S. Pat. No. 7,274,015 to Miller et al., entitled “Capacitive discharge plasma ion source”, and U.S. Pat. No. 7,157,721 to Blanchard, entitled “Coupled ionization apparatus and methods”). The sample gas is typically passed through the corona discharge region in order to utilize the accelerated electrons to produce the ionization. Corona discharge ion sources may exist as a single pair of electrodes or as a plurality of electrodes (see, for example, U.S. Pat. No. 7,326,926 to Wang, entitled “Corona discharge ionization sources for mass spectrometric and ion mobility spectrometric analysis of gas-phase chemical species”) for greater reliability and lifetime. All of the above-noted references are incorporated herein by reference.
Ion sources that are radioactive require expensive and inconvenient disposal of the source material when no longer needed for service. X-ray sources have a limited lifetime of operation and are expensive to replace. Photoelectric ion sources do not readily produce both positive and negative ions. Ultraviolet lamps are relatively large due to the expensive UV-transmitting envelope. Ultraviolet lamp envelopes are electrically insulating and can become charged with static, affecting the nearby electric field and attracting a particulate coating that obscures the emission of light. Ultraviolet lamp envelopes are difficult to clean, because common solvents leave an ultraviolet-blocking surface coating when they dry. Corona sources may have lifetime limitations or have an output of ions in both quantity and species that is sensitive to the instantaneous path of the discharge.
Accordingly, it would be desirable to provide a system that addresses the above-noted issues and improves the production of ions for use with a chemical detector.